Polyphase AC Induction Motors

How do you overcome the problem? The bottom of Figure 6-7 shows one key. If you introduce a second winding that is physically at right angles to the main stator winding, you induce a rotor current out of phase with the main rotor current that is sufficient to start the motor. This split phase induction motor design—or some variation of it—is the one you are most likely to encounter on typical smaller motors that power fans, pumps, shop motors, etc. To maximize the electrical phase difference between the two windings, the resistance of the starting winding is much higher and its induction is much lower than the running winding. To minimize excessive power dissipation and possible temperature rise after the motor is up and running, a shaft-mounted centrifugal switch is connected in series with the starting winding that opens at about three-fourths of synchronous speed. Figure 6-7 looks like a representation of a shunt motor: the smaller split-phase induction motor speed characteristics look like those of DC shunt motors, but their starting torque is much greater.

The most common split-phase induction motor is one that uses a capacitor-start, also shown in Figure 6-7. The capacitor automatically provides a greater electrical phase difference than inductive windings. This greater phase difference—nearly 90 electrical degrees—also gives capacitor-start split-phase induction motors a much higher starting torque (three to five times rated torque is common). The principle was discovered quite early by Charles Steinmetz and others, but capacitor technology had to catch up before it could be widely introduced on production motors. Capacitor-start design variations include two types: separate starting and running capacitors; and permanent capacitor with no centrifugal cutout switch. The two-capacitor approach brings you the best of both the starting and the running worlds; the permanent capacitor type gives you superior speed control during operation at the expense of lower starting torque. The other common split-phase induction motor design, called shaded pole, applies mostly to smaller motors; you are more likely to find it in your electric alarm clock than in your EV, so we’ll skip it here.

Polyphase AC Induction Motors

Polyphase means more than one phase. AC is the prevailing mode of electrical distribution. Single-phase 208V to neutral from a three-phase transformer on the pole is the most prevalent form found in your home and office. The phase voltage that comes from the pole is 240V. These are widely available in nearly every city in the industrialized world. If one phase is good, then three phases are better, right? Well, usually. Stationary threephase electric induction motors are inherently self-starting and highly efficient, and electricity is conveniently available.

Three-phase AC connected to the stator windings of a three-phase AC induction motor produces currents that look like those shown at the top of Figure 6-8—they are of the same amplitude, but 120 degrees out of phase with one another.

As in a DC motor, power and torque are also a function of current in an induction motor. Because the current is equal to the voltage divided by the motor reactance, at any given voltage, current is a function of stator, rotor, and magnetizing reactances that change as a function of frequency. The top of Figure 6-9 shows this at a glance.

The characteristic induction motor torque to slip graph, shown in Figure 6-9 for both its motor and generator operating regions, offers insight into induction motor operation. If an induction motor is started at no load, it quickly comes up to a speed that might only be a fraction of 1 percent less than its synchronous speed. When a load is applied, speed decreases, thereby increasing slip; an increased torque is generated to

Figure 6-8 Polyphase AC motor operation summarized.

meet the load up to the area of full load torque, and far beyond it up to the maximum torque point (a maximum torque of 350 percent–rated torque is typical).

Speed and torque are relatively easy to handle and determine in an induction motor. So are reversing and regenerative braking. If you reverse the phase sequence of its stator supply (that is, reverse one of the windings), the rotating magnetic field of the stator is reversed, and the motor develops negative torque and goes into generator action, quickly bringing the motor to a stop and reversing direction (see Figure 6-10). Regenerative braking action—pumping power back into the source—is readily accomplished with induction motors. How much regenerative braking you apply creates braking (moves the steady state induction motor operating point down the

Figure 6-9 Polyphase AC motor’s unique speed, torque, and slip characteristics versus voltage and frequency.

Figure 6-10 AC electric motor (Courtesy of Zero Emission Vehicles of Australia).